(i) Field of the Invention
The present invention relates to fiber optic communications systems in general and more particularly, to the apparatus and method which improve its transmission characteristics.
(ii) Prior Art
The proliferation of data handling networks has created a need for more effective and efficient interconnecting transmission systems. A typical data handling network includes a plurality of data handling devices (such as work stations, computers, telephones, etc.) interconnected by a transmission system. Each device includes a transmitter which outputs digital data that is transmitted over the transmission system to the receiver of a receiving device. The transmission system may be a physical medium (e.g., wire, optical fiber, etc.) or non-physical medium which is often referred to as wireless communication.
The present invention relates to a communications network in which fiber optics is the interconnecting medium. This type of network is important because it has the capability of transmitting large volumes of data at relatively high speed, say 100 or greater million bits per sec. One of the problems which is associated with fiber optic networks is intersymbol interference which limits the length of an optical fiber link and the speed of data on the link. For example, a typical laser driven single-mode fiber system operating at 2.5 Gb/s would be limited to 1.7 Km. This means that in order for data to be reliably recovered at a receiver, the maximum spacing between the transmitter and the receiver is approximately 1.7 Km. As is well known to those skilled in the communications technology, a fiber link of 1.7 Km is too short for either a local area network or metropolitan area network. Of course, if the data rate drops, one may transmit and recover data reliably over a longer optical fiber link.
A major cause of intersymbol interference is chromatic dispersion. The chromatic dispersion is an optical phenomenon in which light is propagated at different speeds through the fiber. The speed is related to the wavelength of the light. The power in an optical pulse is distributed over a band of wavelengths which are representative of the spectral width of the source which generates the pulse. Therefore, when a light source such as a laser, generates and launches a pulse into an optical fiber link, the power is distributed over a band of wavelengths. Because each wavelength travels down the fiber at different speeds (chromatic dispersion), the power arrives at different times at the receiver. Stated another way, some of the power in the pulse arrives at the receiver early and some arrives late. The result is that a sharply defined pulse traveling down the fiber is spread out in time to overlap other pulses at the receiver.
FIG. 1 is a graphical representation of the "spread-out" phenomenon, at the receiver of a receiving unit, which the inventors recognized and solved with the present invention. For purposes of this application, the overlapping of the pulse is called intersymbol interference. The figure shows a grouping of overlapped pulses in the time domain. In particular, time is represented on the horizontal axis and pulses 10, 12, and 14 are traveling in the direction of arrow 16. The broadening of the pulses causes overlap in both leading and trailing edges. The overlapping areas may be viewed as composite signals with energy that cannot be easily correlated with its associated pulses. Stated another way, the overlapping energy from adjacent pulses interferes with proper detection of the data.
Since adjacent pulses in a digital data stream must be separated sufficiently in time to assure accurate detection of each pulse, the data rate is limited by the wavelength dependent delay down the fiber interacting with the band of wavelengths produced by the source. Therefore, if the light source has a narrow spectral width, the intersymbol interference is minimized. Laser light sources are devices with narrow spectral widths and are widely used in fiber optic communications networks. However, even with the use of the best quality lasers and/or optical fiber, as the data rate and/or link length increase, intersymbol interference limits the data transmission speed.
U.S. Pat. Nos. 4,538,283 and 4,555,789 disclose a type of intersymbol interference which causes the "eye" opening exhibited by pulses at the receiver to close, beginning at the corners and progressing towards the center. This phenomenon affects the sensitivity of the receiver to differentiate between a "0" and a "1". An equalizer circuit is used to restore the sensitivity of the receiver. It should be noted that these solutions are done in the electrical domain and are limited by the proximity of the carrier frequency to the data rate.